The FPA-4 represents the final FA design, combining the efforts of MLW (assembly), the Canadian General Electric Company Ltd. (electrical equipment), and the Dominion Engineering Works Ltd. of Lachine, Quebec (diesel prime mover). While MLW had produced FA-1/FB-1 and FA-2/FB-2 (and the passenger FPA-2/FPB-2) models for both the CN and CPR, only MLW produced the FPA-4/FPB-4 model. In the mid-1950's, as the march to 100% dieselization in the US was nearing completion, the Canadian railroads still had a way to go. Therefore, MLW's introduction of the Model 251 powered locomotive line found a more favorable market opportunity. In 1958, MLW introduced the passenger cab unit version of the RS-18, the FPA-4/FPB-4. At the time, the CN was in the midst of its final drive to dieselize its long distance and intercity passenger service, acquiring large numbers of locomotives from both GMD and MLW. The CN obtained 34 FPA-4's and 12 FPB-4's in 1958-59. Without a doubt, the most successful FA/FB's of all, these 90 mph passenger locomotives ran for 30 years on the high speed Montreal-Toronto corridor, from Montreal to the Maritimes and to the west from Toronto to Windsor and Sarnia. When VIA Rail Canada took over intercity passenger service, the FPA-4/FPB-4 operations expanded to CPR lines in Ontario to Sudbury and across Maine to St, John, NB. Their high speed prowess, though, could best be seen in Montreal-Toronto service. The CNR's first purchase of MLW passenger diesels consisted of six FPA-2's and six FPB-2's in 1955. In 1959, two FPA-2's and two FPB-2's were upgraded with Model 251B diesel engines, and renumbered immediately below the FPA-4/FPB-4 number blocks of 6760/6860 and above. The only external difference between the upgraded FPA-2/FPB-2's and the FPA-4/FPB-4's was the difference in the air brake system intercooler position. The FPA-2/FPB-2's intercooler heat exchanger was positioned on the unit's rear, as on all FA-2/FB-2's. The intercoolers on the FPA-4/FPB-4 were on each side immediately below the radiator housing, as indicated by the short Farr-Aire grill panel, and a spotting feature.